Casting defects

Defects in casting procedures in dentistry

1. CASTING DEFECTS
DR. AATIF KHAN
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2. INTRODUCTION
•A Casting defect is basically an error that occurs during the fabrication of a
prosthesis. (Mahalaxmi)
•Mostly caused due to the carelessness or ignorance of the operator and these can be
avoided if the fundamentals and principles of casting procedures are followed strictly.
•An unsuccessful casting results in considerable trouble and loss of time.
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3. TYPE OF CASTING DEFECTS
I.DISTORTION
II.SURFACE ROUGHNESS & IRREGULARITIES
III.POROSITY
IV.INCOMPLETE OR MISSING DETAIL
V.SURFACE DISCOLOURATION
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4. 1. DISTORTION
•Distortion leads to a change in the shape and size
of a prosthesis.
•Wax distortion is the most serious problem that can
occur during the forming and removal of the
pattern from the mouth or die.
4DISTORTION

5. Causes:
•Mishandling of the wax pattern.
•Pressure from setting and hygroscopic expansion
of investment material.
•Contraction on cooling.
•Thickness of the wax.
Marginal discrepancy due to distortion of
wax pattern/ uneven expansion.
5DISTORTION

6. PREVENTION -
•Use of Hard wax.
•Place the wax in increments and at uniform
temperature.
•Do not over heat wax or allow the wax to cool
suddenly.
•Carve with sharp instruments.
6DISTORTION

7. PREVENTION -
•Careful handling of wax pattern while
removing it from the die/mouth.
•Invest wax pattern immediately to prevent
warpage.
•Place it in the centre of the casting ring.
7DISTORTION

8. 2. SURFACE ROUGHNESS AND
IRREGULARITIES
•Surface roughness is defined as relatively finely spaced surface imperfections whose
height, width, and direction establish the predominant surface pattern.
•Surface irregularities are isolated imperfections, such as nodules, that are not
characteristic of the entire surface area.
•Excessive roughness or irregularities on the outer surface of the casting necessitate
additional finishing and polishing, whereas irregularities on the cavity surface
prevent proper seating of an otherwise accurate casting.
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9. SURFACE ROUGHNESS
9SURFACE ROUGHNESS

10. CAUSES OF SURFACE ROUGHNESS -
•Particle size of the investment and its ability to reproduce the wax pattern in
microscopic detail.
•Binder-Silica Ratioinfluences the surface texture of the casting. Coarse silica causes
surface roughness.
•Prolonged heating of investmentmaterial (Gypsum) →Breakdown of investment at
excessive burnout temperature.
•HighL/Pratio.
•Excessof wettingagent –dilutes investment.
•When foreign substances get into the mold.
•Impact of highly heated molten alloy on the moldwall. (Attacks moldsurface)
10SURFACE ROUGHNESS

11. SURFACE
IRREGULARITIES
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12. TYPES SURFACE IRREGULARITIES
1.Nodules
2.Water films
3.Fins
12SURFACE IRREGULARITIES

13. 1. NODULES
•Small nodules on a casting are caused by air bubbles that become attached
to the pattern during or after investing.
•Nodules on margins or on internal surfaces might alter the fit of the
casting.
•Improper brush technique –multiple nodules.
Prevention:
1.Use the vacuum investing technique.
2.Manual-mixing mixer with vibration.
3.Wetting agent –thin layer & air dried.
13SURFACE IRREGULARITIES

14. 2. WATER FILMS
•Since waxis repellentto water, a water filmmay form
irregularly over the surface.
•This type of surface irregularity appears as minute ridges or
veins.
Causes:
1.Patternis slightly moved or vibrated after investing.
2.No intimate contactof the investment with the pattern
while painting.
3.High P/L ratio.
•Prevention–Use of wetting agent.
14SURFACE IRREGULARITIES

15. 3. FINS
•Thin metal extensions observed in castings.
•Fins occur when cracksare produced in the investment
that radiate out from the surface of the pattern .
•Molten alloy flows into the cracks forming thin fins on the
casting.
•Increases the time required for finishing.
15SURFACE IRREGULARITIESSURFACE IRREGULARITIES -FINS

16. CAUSES OF FIN FORMATION
1. Improper positioning of the pattern.
•Positioning of several patterns too close and in the same plane in the mold, lead to
formation of fins.
The expansion of the wax is much greater than that of the investment, causing
breakdown or cracking of the investment if the spacing between patterns is less than
3mm.
16SURFACE IRREGULARITIESSURFACE IRREGULARITIES -FINS

17. CAUSES OF FIN FORMATION
•2. Patterns placed too near the edge of the investment.
•If too little investment covers the wax patterns, the alloy is more likely to break
through the mold.
18SURFACE IRREGULARITIESSURFACE IRREGULARITIES -FINS

18. •Correct position for a single coping in the
investment ring.
•The periphery should be 6 mm from the end of
a standard investment ring.
19SURFACE IRREGULARITIESSURFACE IRREGULARITIES -FINS

19. CAUSES OF FIN FORMATION
3. High Heating Rates –
•High heating rate of investment material –outer layer heats up faster.
•Outer layer tends to expand –held back by inner cooler part.
•Outer layer →Compressive stress, Inner layer →Tensile stress.
•Investment (brittle material) –cracks under tensile stress.
•Cracks filled with alloy Fins.
20SURFACE IRREGULARITIESSURFACE IRREGULARITIES -FINS

20. PREVENTION
•Ideally, 60 mins should elapse during the heating of the investment filled ring from
room temperature to 700º C.
•The greater the bulk of the investment, the more slow it should be heated.
21SURFACE IRREGULARITIESSURFACE IRREGULARITIES -FINS

21. CAUSES OF FIN FORMATON
4. Premature Heating -
•If setting is not complete at the time when ring is placed in the oven, the moldmay
become weak and unable to withstand steam pressure during burnout.
Prevention :
•Burnout should be initiated only after the recommended setting time.
22SURFACE IRREGULARITIESSURFACE IRREGULARITIES -FINS

22. CAUSES OF FIN FORMATION
5. Casting Pressure -
•Too high pressure during casting causes fins.
Prevention :
•Casting should provide enough force to cause the liquid alloy to flow onto the heated
mold.
•Adjust the casting machine to the requirements of each alloy.
23SURFACE IRREGULARITIES -FINS

23. 3. POROSITIES
Solidification defects –
A. Localized shrinkage porosity.
B. Micro-porosity.
C. Suck-back porosity.
Trapped gases –
A. Pinhole porosity.
B. Gas inclusions.
C. Subsurface porosity.
D. Back pressure porosity.
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24. POROSITIES
DUE TO SOLIDIFICATION DEFECTS -
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25. A. LOCALIZED SHRINKAGE POROSITY
•Premature terminationof the flow of molten metal during solidification.
•Linear contraction of noble metal alloys from a liquid to a solid state is at least 1.25%.
•If the sprue freezes in its cross section beforethis flow is completed to the casting
proper, a localized shrinkage void will occur in the last portionof the casting that
solidifies.
26LOCALIZED SHRINKAGE POROSITY

26. •Occurs usually near the sprue-casting junction as it is the last and the bulkiest portion
of the casting to solidify.
27LOCALIZED SHRINKAGE POROSITY
Solidifies first
Solidifies later
Solidifies last
Normal metal flow and order of solidification Changes in solidification leading to localized shrinkage

27. PREVENTION
Use of reservoir –
•Attach the large reservoir in the sprue
of thickness more than the thickest portion of
the pattern and as close as possible to the pattern.
Alloy that fills the restoration will
solidify first.
As the molten metal solidifies, it
shrinks and creates a vacuum
Vacuum will draw additional metal
from an adjacent source →The
reservoir.
28LOCALIZED SHRINKAGE POROSITY

28. PREVENTION
•Position the wax pattern in a “cold zone” of the investment
mold and the reservoirin the “heat centre” of the casting ring.
•Investment covering the patterns -no more than ¼ inch (6mm)
•The proper length of sprue is
important to keep the pattern away
from heat center close to the end
of the casting ring.
29LOCALIZED SHRINKAGE POROSITY
6mm

29. B. MICROPOROSITY
•Generally present in fine-grain alloycastings when the
solidificationis too rapid for the microvoidsto segregate
to the liquid pool.
•This premature solidification causes the formation of
small, irregular voids.
•Occurs from rapid solidification if the moldor casting
temperature is too low.
•Unfortunately, this type of defect is not detectable
unless the casting is sectioned.
30SOLIDIFICATION DEFECT

30. C. SUCK BACK POROSITY
•Occurs in the interior of a crown near thearea of the sprue, if a hot spot has been created by the
hot metal impinging from the sprue channel on a point of the moldwall.
•Hot spotcauses the local region to freeze last and results in suck-back porosity.
•It often occurs at an occlusoaxialline angle or incisoaxialline angle that is not well roundedand
when sprueis attached at 90 degrees to the pattern.
31SUCK BACK POROSITY

31. C. SUCK BACK POROSITY
32SUCK BACK POROSITY

32. Hot spot
Impedance to flow
Sprue
90º
45º
Sprue former
Continuity of flow
Prevention –Flare the point of sprue attachment
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33. •If gas cannot escape from the internal surface of the crown mold it may result in suck-back in
the sprue rod. Venting can prevent this from occurring.
34SUCK BACK POROSITY

34. POROSITIES
DUE TO TRAPPED GASES -
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35. 1. PINHOLE POROSITY
•Many metals dissolve or occlude gases in their molten state e.g. both
copper and silver dissolve O2 in large amount in liquid state.
•Molten platinumand palladium have a strong affinityfor hydrogenas well
as oxygen.
•On solidificationof metal absorbed gases are expelledresulting in pinhole
porosities.
•Castings that are severely contaminated with gases are usually blackwhen
removed and do not clean easily on pickling.
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36. 2. GAS INCLUSION POROSITY
•The gas inclusion porosities are usually much larger than pinhole porosity.
•The larger spherical voids are caused by gasthat is mechanically trapped by the
molten metal in the moldor by gas that is incorporated during the casting procedure.
37GAS INCLUSION POROSITY

37. The gas could be occluded from poorly adjusted torch flame or by use of mixing or
oxidizing zone of flame rather than reducing zone.
38GAS INCLUSION POROSITY

38. 3. SUBSURFACE POROSITY
When the molten metal comes in contactwith the low temperature mold, the outer layer
coming in contact with the moldwall solidifiessuddenly and makes a skin of solid metal
which is tenaciously adherent to the moldwall. When the inner layer shrinks ,the outer
covering of solid metal cannot be dragged along with it. This leads to subsurface porosity.
Prevention-Can be diminished by controlling the rate at which molten metal enters the
mold.
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39. D. BACK PRESSURE POROSITY
•Air/Gases present in the moldcavity are pushed out by molten metal.
•Presence of air in the cavity will not allow the metal to flow.
Air in moldescapes by:
1.Pores in the investment.
2.By pressure gradient that displaces the air pocket toward the end of the investment
via the molten sprue and button.
40BACK PRESSURE POROSITY

40. •Back pressure effects are caused by an inability of air or other gases within the mold
to escape, making a way for the alloy.
•As the liquid enters the mould through the sprue, the air trapped in the moldis
compressedat the extremities, which exerts back pressure preventing the alloy liquid
to occupy this region.
•Usually found in a “pocket” at the occlusal surface of a crown or mesio-occlusal-distal
casting.
41BACK PRESSURE POROSITY

41. A casting which has been subjected to back pressure is rounded at the
edges and lacking in detail.
42BACK PRESSURE POROSITY
Rounded edges due
to back pressure.

42. Causes:
•Dense modern investments.
•Increased distance between the pattern and end of the casting ring.
•Inadequate casting and moldtemperature.
•Clogging of moldwith residual carbon.
43BACK PRESSURE POROSITY

43. Prevention:
•Proper burnout temperature.
•Adequate moldand casting temperature.
•High casting pressure.
•Proper L/P ratio.
•Thickness of investment between pattern and end of casting ring not greater than
6mm.
44BACK PRESSURE POROSITY

44. INCOMPLETE CASTING
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45. 4. INCOMPLETE CASTING
Causes:
•Insufficient venting of the mold.
•High viscosity of the fused metal.
•Insufficient casting pressure.
•Incomplete elimination of wax.
•Insufficient alloy.
•Blocking due to pre-solidification in sprue.
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46. SURFACE
DISCOLORATION
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47. 5. SURFACE DISCOLORATION -
•Prolonged heating of the mold cause disintegration of gypsum bonded investment and
products of decomposition are sulphurcompounds that contaminate the alloy to the
extent that surface texture is affected. Does not respond to pickling.
•Surface discoloration also results from high sulphurcontent of flame torch.
•Interaction of molten alloy with Sulphur→Black/Grey layer on the surface of gold
alloys that is brittle and doesn't clean readily during pickling.
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51. THANK YOU
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